1. Field of the Invention
The present invention relates to an antenna structure preferred for a wireless communication device such as a cellular phone with a plurality of frequencies for transmission/reception, as well as to an antenna adaptable to a plurality of frequencies, a method of adjusting resonance frequency thereof, and a wires communication device.
2. Description of the Related Art
Antennas used for wireless communication such as cellular phones increasingly use a multi-band, such as a dual-band or triple-band, having a communication band made up of a plurality of frequencies. To avoid installing an antenna for each band in a communication device, it is necessary to have a multi-band function that supports a plurality of frequencies with one antenna, and since compactness and design of the device are deteriorated by projecting an antenna, it is requested to incorporate the antenna within a housing.
The multi-band antennas include a dual-band inverted F antenna with an element for each different target frequency (National Publication of Translated Version No. 2002-520935 (paragraph No. 0021, FIG. 3, FIG. 4, etc.)) and an antenna using two inverted F antennas to support a triple-band or more (Japanese Patent Application Laid-Open Publication No. 2003-124730 (paragraph No. 0016, FIG. 1, FIG. 2, etc.)). National Publication of Translated Version No. 2002-520935 discloses that load resistance is inserted to make an antenna broadband, and Japanese Patent Application Laid-Open Publication No. 2003-124730 discloses that an antenna is short-circuited to a substrate GND by a switching device.
By the way, with respect to an antenna structure used with a cellular phone, for example, a multi-band compatible antenna 2 shown in FIGS. 1 and 2 is a monopole antenna, including an element 4 corresponding to a target frequency f1 and an element 6 corresponding to a target frequency f2 (>f1), and is installed on a circuit substrate 10 of a cellular phone 8. Such an antenna 2 has a VSWR (Voltage Standing Wave Ratio) characteristic shown in FIG. 3 and has VSWR=3 or less at the target frequency f1 (e.g., 80 [MHz] band) and the target frequency f2 (e.g., 2 [GHz] band). Therefore, the antenna 2 realizes a structure that exposes portions of the elements 4, 6 outside of the housing (semi-built-in structure) and a multi-band compatibility by folding the elements 4, 6 into an L-shape.
However, when attempting to completely house the antenna 2 constituted by such a monopole antenna within the housing, characteristic degradation is caused, the element 6 for the target frequency f2 is interfered by the element 4, and the target frequency f2 is prevented from supporting a broadband. Therefore, such an antenna 2 is not suitable to be housed in the housing completely and is not suitable for expanding the target frequency f2 (e.g., 1.7 [GHz] band).
When a planar inverted F antenna (PIFA) is used for the purpose of completely housing the antenna within an antenna-mounted device, a dual-band inverted F antenna may be constituted which has elements for a target frequency f1 and a target frequency f2 (National Publication of Translated Version No. 2002-520935). In general, an inverted F antenna has a narrow band and causes no problem in practical use if the bandwidth is on the order of 150 [MHz] in 2.4 [GHz] band, however, problems are posed in practical use if the bandwidth is expanded. If load resistance is inserted to support a broadband, the resistance consumes electric power and the radiation efficiency of the antenna is reduced.
When a triple-band or more is supported with the use of two inverted F antennas and a switching device is provided for short-circuiting each antenna to the substrate GND (Japanese Patent Application Laid-Open Publication No. 2003-124730), costs are increased by providing the switching device although a plurality of frequency can be supported.
National Publication of Translated Version No. 2002-520935 and Japanese Patent Application Laid-Open Publication No. 2003-124730 do not indicate or disclose such problems and do not have a configuration or idea for solving the problems.